Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In power Doppler imaging, the power contained in the returned Doppler signal is displayed. Although the following disclosure refers predominantly to B-mode imaging for the sake of brevity, the present invention applies to any mode of ultrasound imaging.
The basic signal processing chain in the conventional B mode is depicted in FIG. 1. An ultrasound transducer array 2 is activated to transmit an acoustic burst along a scan line. The return RF signals are detected by the transducer elements and then formed into a receive beam by the beamformer 4. The beamformer output data (I/Q or RF) for each scan line is passed through a B-mode processing chain 6 which includes equalization filtering, envelope detection and logarithmic compression. Depending on the scan geometry, up to a few hundred vectors may be used to form a single acoustic image frame. To smooth the temporal transition from one acoustic frame to the next, some acoustic frame averaging 8 may be performed before scan conversion. For a sector scan, compressed images in R-.theta. format are converted by the scan converter 10 into X-Y format for display. On some systems, frame averaging may be performed on the X-Y data (indicated by dashed block 12) rather than the acoustic frames before scan conversion, and sometimes duplicate video frames may be inserted between acoustic frames in order to achieve a given video display frame rate (typically 30 Hz). The scan-converted frames are passed on to a video processor 14, which basically maps the scan-converted data to a display gray or color map for video display.
System control is centered in a host computer 20, which accepts operator inputs through an operator interface 22 (e.g., keyboard and track-ball) and in turn controls the various subsystems. (In FIG. 1, the system control lines from the host computer to the various subsystems have been omitted for the sake of simplicity.) During imaging, a long sequence of the most recent images are stored and continuously updated automatically in a cine memory 16. Some systems are designed to save the R-.theta. acoustic images (this data path is indicated by the dashed line in FIG. 1), while other systems store the X-Y video images. The image loop stored in cine memory 16 can be reviewed on the display monitor via track-ball control (interface 22), and a section of the image loop can be selected for hard disk storage. For an ultrasound scanner with free-hand three-dimensional imaging capability, the selected image sequence stored in cine memory 16 is transferred to the host computer 20 for three-dimensional reconstruction. The result is written back into another portion of the cine memory or to scan converter memory, from where it is sent to the display system 18 via video processor 14.
Referring to FIG. 2, the scan converter 10 comprises an acoustic line memory 24 and an XY display memory 26. A separate acoustic line memory (not shown) is provided for color flow acoustic samples.! The B-mode data stored in polar coordinate (R-.theta.) sector format in acoustic line memory 24 is transformed to appropriately scaled Cartesian coordinate intensity data, which is stored in XY display memory 26. Each image frame out of XY display memory 26 is sent to both the video processor 14 and the cine memory 16.
A multiplicity of successive frames of B-mode data are stored in cine memory 16 on a first-in, first-out basis. The cine memory is like a circular image buffer that runs in the background, continually capturing image data that is displayed in real time to the user. When the user freezes the system (by depressing the FREEZE key on the interface 22), the user has the capability to view image data previously captured in cine memory.
The conventional system has the capability to superimpose graphical symbols on any ultrasound image. The superimposition of graphics on the image frame is accomplished in the video processor 14, which receives the ultrasound image frame from the XY display memory 26 and the graphics data from a graphics display memory 34. The graphics data is processed and input into the graphics display memory 34 by a graphics processor 36, which is synchronized with the other subsystems by the host computer 20.
The host computer 20 comprises a central processing unit (CPU) 28 and a random access memory 30. The CPU 28 has memory for storing routines used in transforming an acquired volume of intensity data into a multiplicity of three-dimensional projection images taken at different angles. The CPU 28 controls the X-Y memory 26 and the cine memory 16 via the system control bus 32. In particular, the CPU 28 controls the flow of data from the acoustic line memory 24 or from the X-Y memory 26 of the scan converter 10 to the video processor 14 and to the cine memory 16, and from the cine memory to the video processor 14 and to the CPU 28 itself. Each frame of imaging data, representing one of a multiplicity of scans or slices through the object being examined, is stored sequentially in the acoustic line memory 24, in the X-Y memory 26 and in the video processor 14. In parallel, image frames from either the acoustic line memory or the X-Y memory are stored in cine memory 16. A stack of frames, representing the scanned object volume, is stored in cine memory 16, forming a data volume.
Two-dimensional ultrasound images are often hard to interpret due to the inability of the observer to visualize the two-dimensional representation of the anatomy being scanned. However, if the ultrasound probe is swept over an area of interest and two-dimensional images are accumulated to form a three-dimensional data volume, the anatomy becomes much easier to visualize for both the trained and untrained observer.
In order to generate three-dimensional images, the CPU 28 can transform a source data volume retrieved from cine memory 16 into an imaging plane data set. The successive transformations may involve a variety of projection techniques such as maximum, minimum, composite, surface or averaged projections made at angular increments, e.g., at 100 intervals, within a range of angles, e.g., +90.degree. to -90.degree.. Each pixel in the projected image includes the transformed data derived by projection onto a given image plane.
In free-hand three-dimensional ultrasound scans, a transducer array (1 D to 1.5 D) is translated in the elevation direction to acquire a substantially parallel set of image planes through the anatomy of interest. These images can be stored in the cine memory and later retrieved by the system computer for three-dimensional reconstruction. If the spacings between image frames are known, then the three-dimensional volume can be reconstructed with the correct aspect ratio between the out-of-plane and scan plane dimensions. If, however, the estimates of the interslice spacing are poor, significant geometric distortion of the three-dimensional object can result.
A conventional ultrasound scanner collects B-mode, color and power Doppler data in a cine memory on a continuous basis. As the probe is swept over an area of the anatomy, using either a free-hand scanning technique or a mechanical probe mover of some sort, a three-dimensional volume is stored in the cine memory. The distance the probe was translated may be determined by a number of techniques. The user can provide an estimate of the distance swept. If the probe is moved at a constant rate by a probe mover, the distance can easily be determined. Alternatively, a position sensor can be attached to the probe to determine the position of each slice. Markers on the anatomy or within the data could also provide the required position information. Yet another way would be to estimate the scan plane displacements directly from the degree of speckle decorrelation between successive image frames. Once the data volume has been acquired, the central processing unit can then provide three-dimensional projections of the data as well as arbitrary slices through the data volume.
Referring to FIG. 3, if the ultrasound probe 2' is swept (arrow 38 indicates a linear sweep) over an area of a body (either by hand or by a probe mover), such that the interslice spacing is known, and the slices 40 are stored in memory, a three-dimensional data volume 42 can be acquired. The data volume can be processed (e.g., using projection onto an imaging plane) to form a three-dimensional view of the area of interest. In addition, the data can be reformatted to produce an individual slice 44 at an arbitrary angle (see FIG. 4), thus allowing the user to get the exact view desired regardless of the anatomy under investigation. Algorithms for producing three-dimensional projections of two-dimensional data are well known, as are techniques for reformatting data to produce arbitrary slices through a data set. The problem that arises is how to display the information such that it is easy for the observer to easily relate the two-dimensional slice to the three-dimensional anatomy.